Remove FF zoekt_search_api

What does this MR do and why?

This change removes a feature flag called zoekt_search_api that was previously used to control whether the Zoekt search engine could be used through the API. The feature flag and all its related code have been completely removed, which means Zoekt search is now always available through the API when other conditions are met. This simplifies the codebase by eliminating the conditional logic that was checking this flag, and removes all the associated test cases that were verifying the flag's behavior. The change also removes the feature flag configuration file itself, indicating this feature has been fully rolled out and no longer needs to be toggled on or off.

References

gitlab-com/gl-infra/production#17522 (closed)

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